Polymer derived from acrylonitrile

ABSTRACT

This invention relates to poly(acrylonitrile) homo- or co-polymer having a number average molecular weight (M n ) of at least 200,000 g/mol and a dispersity ( ) of less than 1.3

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theU.S. Provisional Application No. 61/799,506, filed Mar. 15, 2013, thedisclosure of the prior application of which is hereby incorporated inits entirety by reference.

FIELD OF THE INVENTION

The present invention relates in general to polymer derived fromacrylonitrile monomer. In particular, the invention relates topoly(acrylonitrile) homo- and co-polymers, to a method for producing thesame, to carbon fibre comprising carbonised residue of the polymer, andto a method for producing the carbon fibre. The poly(acrylonitrile)homo- and co-polymers according to the invention have been found to beparticularly suitable for use in the manufacture carbon fibre, and itwill therefore be convenient to describe the invention with particularreference to this application. However, it is to be understood that thepoly(acrylonitrile) homo- and co-polymers according to the invention maybe used in a variety of other applications.

BACKGROUND OF THE INVENTION

Poly(acrylonitrile) (PAN) is a synthetic, semi crystalline organicpolymer that typically has a linear structure of general formula(C₃H₃N)_(n). Commercially, PAN is often produced in the form of aco-polymer with one or more other ethylenically unsaturated monomers.

Although PAN-based polymers are generally thermoplastic, they may not gothrough a molten transition under normal conditions, but rather degradeprior to melting.

PAN-based polymers are particularly versatile and are used tomanufacture numerous products including filtration membranes, and fibreshaving a diverse range of applications.

PAN-based fibres have been found to be particularly suited for use inthe manufacture of carbon fibre. This typically involves first thermallyoxidising PAN-based fibre in air to form oxidised PAN fibre which isthen carbonised at high temperature in an inert atmosphere to make thecarbon fibre.

The properties of carbon fibre, such as high stiffness, high tensilestrength, low weight, high chemical resistance, high temperaturetolerance and low thermal expansion, make it particularly suitable foruse in aerospace, civil engineering, military, automotive and sportingapplications.

In use, carbon fibres are typically combined with a polymer resin toform a composite structure. The resulting composite structures arerenowned for having a very high strength-to-weight ratio.

Because of its unique properties, PAN-based polymer is particularly wellsuited for use in the manufacture of carbon fibre. Despite being usedfor many years as a precursor material in the manufacture of carbonfibre, those skilled in the art will be aware that acrylonitrilepresents numerous challenges in the manufacture of PAN-based polymers.In particular, due to the high reactivity of acrylonitrile and the poorsolubility of PAN-based polymers, the controlled polymerisation ofacrylonitrile has presented a significant challenge to polymerscientists.

PAN-based polymers have traditionally been produced by conventional freeradical polymerisation, the process of which offers limited control overthe molecular weight and dispersity of the resulting polymer.

Increasing the molecular weight while maintaining a low dispersity ofPAN-based polymers is believed to play an important role in enhancingcertain properties of products, such as carbon fibre, derived from thepolymer.

Accordingly, considerable research effort has to date been directedtoward improved methodology for producing PAN-based polymer.

Anionic polymerisation techniques have been applied with some success toproduce relatively well-defined PAN-based polymer. However, thetechniques employed require relatively harsh polymerisation conditionsthat would present as major limitations to adopting the technologycommercially. Furthermore, the technique has only provided for arelatively modest gain in molecular weight over other known techniques.

In more recent times, considerable attention has focussed on using socalled living or controlled radical polymerisation techniques to preparePAN-based polymers. The use of such techniques has resulted in anability to produce PAN-based polymers with an increase molecular weightand a relatively low dispersity. For example atom transfer radicalpolymerisation (ATRP) has been used to prepare PAN with a molecularweight (M_(n)) of about 120,000 g/mol and a dispersity (M_(w)/M_(n)) ofabout 2 (Journal of Polymer Science Part A: Polymer Chemistry Volume 51,Issue 2, pages 340-346, 2013).

Despite offering improvements in the preparation of PAN-based polymers,most of the ATRP techniques developed to date inherently introducetransition metal residues into the resulting polymer. The presence ofsuch transition metal residues can be detrimental in certainapplications for the polymer, for example in the manufacture of carbonfibre.

Other living/controlled radical polymerisation techniques have also beenapplied with some success. For example, ReversibleAddition-Fragmentation chain Transfer (RAFT) polymerisation has beenemployed in the manufacture of the PAN-based polymers. For example,producing PAN with a M_(n) of about 33,000 g/mol and a dispersity of1.29 by RAFT polymerisation was considered to be a significantachievement (Journal of Polymer Science Part A: Polymer Chemistry Volume45, Issue 7, pages 1272-1281, 2007). 2007). In another example,producing PAN with a Mn of 200,000 g/mol and dispersity of 1.7-2.0 byRAFT polymerisation was also claimed as a significant progress (EuropeanPolymer Journal, Volume 44, Pages 1200-1208, 2008).

Those skilled in the art will appreciate that as the M_(n) of a givenpolymer increases it becomes increasingly difficult to maintain a lowdispersity. In the manufacture of PAN-based polymers it has provendifficult to not only produce polymers having a M_(n) of greater than100,000 g/mol but also to maintain the dispersity of the polymer belowabout 1.35. In such an environment an ability to produce PAN-basedpolymers with only a modest increase in M_(n) while maintaining a lowdispersity is considered in the art to be a significant achievement.

Accordingly, there remains an opportunity to develop PAN-based polymersthat exhibit improved properties such as increased molecular weight withlow dispersity.

SUMMARY OF THE INVENTION

The present invention therefore provides poly(acrylonitrile) homo- orco-polymer having a number average molecular weight (M_(n)) of at least200,000 g/mol and a dispersity (

) of less than 1.3.

Considerable research to date has failed to provide for PAN-basedpolymers having a high M_(n) and a low dispersity. It has now been foundthat PAN-based polymers can indeed be prepared having a M_(n) of atleast 200,000 g/mol and a dispersity of less than 1.3. This achievementis believed to represent a significant advance in PAN-based polymertechnology.

By offering a unique molecular structure profile (i.e. high M_(n) andlow dispersity), the PAN-based polymers according to the presentinvention are believed to impart improved properties to polymer andcarbon fibres derived therefrom.

Accordingly, the invention further provides carbon fibre comprisingcarbonised residue of poly(acrylonitrile) homo- or co-polymer having anumber average molecular weight (M_(n)) of at least 200,000 g/mol and adispersity (

) of less than 1.3.

Carbon fibre in accordance with the invention is believed toadvantageously exhibit improved modulus and tensile strength propertiesrelative to equivalent PAN-based polymers having a lower M_(n) and/or ahigher dispersity.

The present invention also provides a method for producingpoly(acrylonitrile) homo- or co-polymers by RAFT polymerisation, themethod comprising polymerising acrylonitrile and optionally one or moreethylenically unsaturated co-monomers under the control of a RAFT agent,wherein the mole ratio of the polymerisable monomers to the RAFT agentis at least 1,000.

Conventional techniques for producing PAN-based polymers by RAFTpolymerisation have typically utilised a mole ratio of monomer to RAFTagent of no more than about 400 and a mole ratio of RAFT agent toinitiator in the range of about 5 to 10. Surprisingly, it has now beenfound that a increase in the monomer to RAFT agent ratio and/or adecrease in the RAFT agent to initiator ratio can advantageouslyfacilitate the production of PAN-based polymers having a high M_(n) anda low dispersity.

In one embodiment, the method according to the invention is forproducing by RAFT polymerisation poly(acrylonitrile) homo- or co-polymerhaving a number average molecular weight (M_(n)) of at least 200,000 anda dispersity (

) of less than 1.3.

Dithiobenzoate and trithiocarbonate RAFT agents have been found to beparticularly well suited to producing the PAN-based polymers accordingto the invention.

In one embodiment, the poly(acrylonitrile) homo- or co-polymer accordingto the invention is a RAFT polymer. In a further embodiment, thepoly(acrylonitrile) homo- or co-polymer is a RAFT polymer and hascovalently bound thereto a dithiobenzoate or trithiocarbonate RAFT agentresidue.

In another embodiment, the RAFT agent used in accordance with the methodof the invention is a dithiobenzoate or trithiocarbonate RAFT agent.

The present invention also provides a method of producing carbon fibre,the method comprising carbonising a fibre comprising poly(acrylonitrile)homo- or co-polymer having a number average molecular weight (M_(n)) ofat least 200,000 and a dispersity (

) of less than 1.3.

In addition to providing for PAN-based polymer having a number averagemolecular weight (M_(n)) of at least 200,000 and a dispersity (

) of less than 1.3, the present invention also advantageously providesfor PAN-based polymer having even lower dispersity.

The present invention therefore also provides poly(acrylonitrile) homo-or co-polymer having a number average molecular weight (M_(n)) of atleast 150,000 g/mol and a dispersity (

) of less than 1.25, or of at least 100,000 g/mol and a dispersity (

) of less than 1.2.

Further aspects and/or embodiments of the invention are discussed inmore detail blow.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will herein be described with reference to the followingnon-limiting drawings in which:

FIG. 1 illustrates the effect of the solvent used on the molecularweight and yield of the PAN obtained under the same polymerisationconditions (AN=5 mol/L, AIBN=1.60×10⁻³ mol/L).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in general to poly(acrylonitrile) homo- orco-polymers. For convenience, these polymers may herein be referred toas “PAN-based” polymers. Those skilled in the art will appreciate that aPAN homo-polymer consists essentially of polymerised acrylonitrilemonomer residues. A PAN co-polymer will comprise polymerised residues ofacrylonitrile and one or more other co-monomer polymerised residues.

By being a PAN co-polymer is meant that the co-polymer will comprisegreater than 50 wt. % of polymerised acrylonitrile monomer residues.

A PAN co-polymer in accordance with the invention will generallycomprise 70-99 wt. % polymerised residue of acrylonitrile and 1-30 wt. %polymerised residue of one or more other ethylenically unsaturatedco-monomers.

PAN-based polymers according to the invention have a number averagemolecular weight (M_(n)) of at least 100,000 g/mol, at least 150,000g/mol, at least 200,000 g/mol. As used herein, the M_(n) of thePAN-based polymers is intended to be that which is measured using GelPermeation Chromatography (GPC), where dimethylacetamide (DMA) is usedas eluent, and polymethylmethacrylate (PMMA) as standards.

In one embodiment, where the dispersity (

) is less than 1.3, the M_(n) of the PAN-based polymers may be at least225,000, or at least 250,000, or at least 275,000, or at least 300,000,or at least 325,000, or at least 350,000, or at least 375,000, or atleast 400,000, or at least 425,000, or at least 450,000 g/mol. In afurther embodiment, the M_(n) ranges from at least 200,000 g/mol toabout 800,000 g/mol, or at least 200,000 g/mol to about 600,000 g/mol.

In another embodiment, where the dispersity (

) is less than 1.25, the M_(n) of the PAN-based polymers may be at least160,000, or at least 170,000, or at least 180,000, or at least 190,000,or at least 200,000 g/mol. In a further embodiment, the M_(n) rangesfrom at least 150,000 g/mol to about 800,000 g/mol, or at least 150,000g/mol to about 600,000 g/mol.

In a further embodiment, where the dispersity (

) is less than 1.20, the M_(n) of the PAN-based polymers may be at least110,000, or at least 120,000, or at least 130,000, or at least 140,000,or at least 150,000 g/mol. In a further embodiment, the M_(n) rangesfrom at least 100,000 g/mol to about 800,000 g/mol, or at least 100,000g/mol to about 600,000 g/mol.

In addition to having a high M_(n), the PAN-based polymers according tothe invention also have a low dispersity (

) of less than 1.3.

As used herein, the dispersity (

) of the PAN-based polymers is determined according to equation (1):

=M _(w) /M _(n)  (1)

where M_(w) is the mass average molecular weight, and M_(n) is as hereindefined.

M_(w) referred to herein is intended to be that as determined by GPC ina similar manner to that outlined above in respect of determining M_(n).

In one embodiment, the PAN-based polymers according to the inventionhave a dispersity no greater than 1.28, or no greater than 1.26, or nogreater than 1.25, or no greater than 1.24, or no greater than 1.22, orno greater than 1.20, or no greater than 1.18, or no greater than 1.16,or no greater than 1.14, or no greater than 1.12. In a furtherembodiment, the PAN-based polymers according to the invention have adispersity ranging from about 1.05 to less than 1.3, or from about 1.1to less than 1.3, or from about 1.12 to less than 1.3.

Where the PAN-based polymer according to the invention is ahomo-polymer, it will be appreciated that the polymer will consistessentially of polymerised residues of acrylonitrile.

Where the PAN-based polymer is a co-polymer, it will generally comprisegreater than 50 wt. % of polymerised acrylonitrile monomer residues,with the remaining polymerised monomers residues being derived from oneor more other co-monomers.

In one embodiment, the PAN-based polymer is a PAN co-polymer comprisingthe polymerised residue of one or more co-monomers other thanacrylonitrile in an amount of no more than about 30 wt. %, or no morethan about 20 wt. %, or no more than about 15 wt. %, or no more thanabout 10 wt. %, or no more than about 8 wt. %, or no more than about 6wt. %, or no more than about wt. %, or no more than about 2 wt. %, or nomore than about 1 wt. %, relative to the total amount of polymerisedmonomer residue.

In a further embodiment, a PAN co-polymer according to the inventioncomprises polymerised co-monomer residue other than acrylonitrile in anamount of about 1 to about 30 wt. %, or about 1 to about 20 wt. %, orabout 1 to about 15 wt. %, or about 1 to about 10 wt. %, or about 1 toabout 8 wt. %, or about 1 to about 6 wt. %, or about 1 to about 4 wt. %,or about 1 to about 3 wt. %, or about 1 to about 2 wt. %, relative tothe total amount of polymerised monomer residue.

RAFT polymerisation has been found to be particularly well suited forproducing PAN-based polymers according to the invention. Accordingly, inone embodiment the PAN-based polymer is a RAFT polymer. In that case,the invention provides a poly(acrylonitrile) RAFT homo- or co-polymer.

As used herein, the expression “RAFT polymer” or “RAFT homo- orco-polymer” is intended to mean a polymer that has been prepared by RAFTpolymerisation (i.e. polymer that is formed by polymerisation of monomerunder the control of a RAFT agent).

Those skilled in the art will appreciate that polymer formed by RAFTpolymerisation will contain (unless it has otherwise been removed) acovalently bound residue of the RAFT agent.)

It has been found that dithiobenzoate and trithiocarbonate RAFT agentsare particularly well suited for producing PAN-based polymers accordingto the invention.

Accordingly, in one embodiment the PAN-based polymer comprises acovalently bound residue of a dithiobenzoate or trithiocarbonate RAFTagent.

Dithiobenzoate and trithiocarbonate RAFT agents that contain a cyanogroup (—CN), a carboxylic acid group (—COOH) or both of such groups havebeen found to be particularly well suited for producing the PAN-basedpolymers according to the invention.

Accordingly, in a further embodiment the PAN-based polymers comprise acyano functionalised, carboxylic acid functionalised, or cyano andcarboxylic acid functionalised dithiobenzoate or trithiocarbonate RAFTagent residue covalently bound thereto.

Specific examples of RAFT agents that are well suited for producing PANco-polymers according to the invention may be selected from:

where the or each R is independently selected from H or CH₃, the or eachR′ is independently selected from H, CH₃, or CN, n=0-15, 4-10, or 10;and m=0-10, 1-5, or 2.

Suitable RAFT agents may also be derived from the following disulphideprecursor compounds:

Accordingly, in one embodiment the PAN-based polymers have covalentlybound thereto a residue of a RAFT agent selected from 1-8 or a precursorcompound selected from 9-11. In a further embodiment, the PAN-basedpolymers have covalently bound thereto a residue of RAFT agent (1) or(5), where R=H or CH₃, R′=CN, m=2, and n=10.

PAN-based polymers according to the invention can be provided in avariety of physical forms. For example, the polymer may be formed intofibre. When in the form of fibre, the PAN-based polymer is well suitedfor use in the manufacture of carbon fibre. Such fibre canadvantageously be used in a conventional carbon fibre manufacturingprocess. Fibre produced from PAN-based polymers according to theinvention has been found to advantageously exhibit uniform diameter.Carbon fibre produced from PAN-based polymers according to the inventionare expected to exhibit improved modulus and tensile properties.

The present invention is therefore also directed PAN-based polymersaccording to the invention in the form of fibre.

The present invention is also directed toward carbon fibre comprisingcarbonised residue of PAN-based polymers according to the invention.

The present invention also provides a method for producingpoly(acrylonitrile) homo- or co-polymer by RAFT polymerisation. Themethod comprises polymerising acrylonitrile and optionally one or moreethylenically unsaturated co-monomers under the control of a RAFT agent.

Of the total amount of monomer polymerised to form the PAN-basedpolymers, acrylonitrile will generally be used in an amount greater than50 wt. %.

There is no particular limitation on the type of co-monomer that may beused in accordance with the invention provided that it can be suitablypolymerised with acrylonitrile. Those skilled in the art will be able toselect suitable co-monomers for this task. Specific examples ofco-monomers that may be used include acrylic acid, methacrylic acid,itaconic acid, allysulfonic acid, and maleic acid, crotonic acid,acrylic acid methylester, methacrylic acid ethylester, ammoniumitaconate, ammonia acrylate, butyl methacrylate, propyl acrylate,stearyl acrylate, and isobutyl methacrylates, methyl methacrylate, ethylmethacrylate, vinyl acetate, or methyl acrylate, vinyl chloride,vinylidine chloride, styrene, acylamide, methacrylamide,3-ammoniumcarboxylate-3-butenoic acid methyl ester, and combinationsthereof.

In one embodiment, the method according to the invention produces a PANco-polymer, and the amount of co-monomer used is no more than about 30wt. %, no more than about 20 wt. %, no more than about 15 wt. %, no morethan about 10 wt. %, or no more than about 8 wt. %, or no more thanabout 6 wt. %, or no more than about 4 wt. %, or no more than about 2wt. %, or no more than about 1 wt. %, relative to the total mol % ofmonomer polymerised. In a further embodiment, the amount of corn-monomerused ranges from about 1 to about 30 wt. %, or 1 to about 20 wt. %, or 1to about 15 wt. %, or 1 to about 10 wt. %, or about 1 to about 8 wt. %,or about 1 to about 6 wt. %, or about 1 to about 4 wt. %, or about 1 toabout 3 wt. %, or about 1 to about 2 wt. %, relative to the total mol %of monomer used.

The method of the invention is well suited to producingpoly(acrylonitrile) homo- or co-polymer having a number averagemolecular weight (M_(n)) of at least 200,000 g/mol and a dispersity (

) of less than 1.3, or poly(acrylonitrile) homo- or co-polymer having anumber average molecular weight (M_(n)) of at least 150,000 g/mol and adispersity (

) of less than 1.25, or of at least 100,000 g/mol and a dispersity (

) of less than 1.2, as described herein.

The monomers used in accordance with the method of the invention arepolymerised under the control of a RAFT agent. By being polymerised“under the control” of the RAFT agent is meant that the monomers arepolymerised via a Reversible Addition-Fragmentation chain Transfer(RAFT) mechanism to form polymer.

RAFT polymerisation of ethylenically unsaturated monomers is describedin WO 98/01478, and in effect is a radical polymerisation technique thatenables polymers to be prepared having a well defined moleculararchitecture and low dispersity.

As previously mentioned, despite RAFT polymerisation being renowned forproviding polymers having a well defined molecular architecture and lowdispersity, imparting such properties to the polymerisation ofacrylonitrile has remained a significant challenge to polymerscientists. Most notably, until recently it has not been possible toprepare PAN-based polymers having a M_(n) greater than about 33,000g/mol and a dispersity of less than 1.29, even by RAFT polymerisation.The present invention surprisingly and advantageously has met thischallenge.

Without wishing to be limited by theory, it is believed that theselection of a particular RAFT agent or precursor thereto may alsofacilitate in providing for PAN-based polymers having a high M_(n) and alow dispersity.

In one embodiment, the RAFT agent or precursor thereto used in themethod of the invention is selected from a dithiobenzoate andtrithiocarbonate RAFT agent.

Dithiobenzoate and trithiocarbonate RAFT agents or precursors theretothat contain a cyano group (—CN), a carboxylic acid group (—COOH) orboth of such groups have been found to be particularly well suited forproducing the PAN-based polymers according to the invention.

Accordingly, in a further embodiment the RAFT agent or precursor theretoused in the method of the invention is selected from a cyanofunctionalised, carboxylic acid functionalised, or cyano and carboxylicacid functionalised dithiobenzoate or trithiocarbonate RAFT agent orRAFT agent precursor compound.

Specific examples of RAFT agents or RAFT agent precursor compounds thatare well suited for use in the method of the invention include thoseherein defined.

In one embodiment the RAFT agent used in the method of the invention isselected compounds I-8. In another embodiment, the RAFT agent used inthe method of the invention is derived from a precursor compoundselected from compounds 9-11. In a further embodiment, the RAFT agentused in the method of the invention is selected from compounds (1) or(5), where R=H or CH₃, R′=CN, m=2, and n=10.

According to the method of the invention, the mole ratio of thepolymerisable monomer used to the RAFT agent used is at least 1,000.Where a RAFT agent precursor compound is used in the method, this ratiorelates to the RAFT agent derived from the precursor compound. Withoutwishing to be limited by theory, it is believed that providing monomerin a significant excess to what would conventionally be employedfacilitates formation of polymer having a high M_(n) and a lowdispersity.

In one embodiment, the mole ratio of the polymerisable monomer to theRAFT agent is at least about 1,500, or at least about 2,000, or at leastabout 2,500, or at least about 3,000, or at least about 3,500, or atleast about 4,000, or at least about 5,500, or at least about 6,000, orat least about 6,500, or at least about 7,000, or at least about 7,500,or at least about 8,000. In a further embodiment the mole ratio ofpolymerisable monomer to the mole ratio of RAFT agents ranges from about1,000 to about 10,000, or about 2,000 to about 8,000, or about 4,000 toabout 8,000.

The polymerisation will usually require initiation from a source of freeradicals. The source of initiating radicals can be provided by anysuitable method of generating free radicals, such as the thermallyinduced homolytic scission of suitable compound(s) (thermal initiatorssuch as peroxides, peroxyesters, or azo compounds), the spontaneousgeneration from monomers (e.g. styrene), redox initiating systems,photochemical initiating systems or high energy radiation such aselectron beam, X— or gamma-radiation. The initiating system is chosensuch that under the reaction conditions there is no substantial adverseinteraction of the initiator or the initiating radicals with themonomers being polymerised.

Thermal initiators are chosen to have an appropriate half life at thetemperature of polymerisation. These initiators can include one or moreof the following compounds:

-   -   2,2′-azobis(isobutyronitrile), 2,2′-azobis(2-cyanobutane),        dimethyl 2,2′-azobis(isobutyrate), 4,4′-azobis(4-cyanovaleric        acid), 1,1′-azobis(cyclohexanecarbonitrile),        2-(t-butylazo)-2-cyanopropane,        2,2′-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide},        2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide],        2,2′-azobis(N,N′-dimethyleneisobutyramidine)dihydrochloride,        2,2′-azobis(2-amidinopropane)dihydrochloride,        2,2′-azobis(N,N′-dimethyleneisobutyramidine),        2,2′-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide},        2,2′-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-ethyl]propionamide},        2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide],        2,2′-azobis(isobutyramide)dihydrate,        2,2′-azobis(2,2,4-trimethylpentane),        2,2′-azobis(2-methylpropane), t-butyl peroxyacetate, t-butyl        peroxybenzoate, t-butyl peroxyneodecanoate, t-butylperoxy        isobutyrate, t-amyl peroxypivalate, t-butyl peroxypivalate,        diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate,        dicumyl peroxide, dibenzoyl peroxide, dilauroyl peroxide,        potassium peroxydisulfate, ammonium peroxydisulfate, di-t-butyl        hyponitrite, dicumyl hyponitrite. This list is not exhaustive.

Photochemical initiator systems are chosen to have the requisitesolubility in the reaction medium and have an appropriate quantum yieldfor radical production under the conditions of the polymerisation.Examples include benzoin derivatives, benzophenone, acyl phosphineoxides, and photo-redox systems.

Redox initiator systems are chosen to have the requisite solubility inthe reaction medium and have an appropriate rate of radical productionunder the conditions of the polymerisation; these initiating systems caninclude, but are not limited to, combinations of the following oxidantsand reductants:

-   -   oxidants: potassium, peroxydisulfate, hydrogen peroxide, t-butyl        hydroperoxide.    -   reductants: iron (II), titanium (III), potassium thiosulfite,        potassium bisulfite.

Other suitable initiating systems are described in recent texts. See,for example, Moad and Solomon “the Chemistry of Free RadicalPolymerisation”, Pergamon, London, 1995, pp 53-95.

Initiators which have an appreciable solubility in a more hydrophilicreaction medium typically include, but are not limited to,4,4-azobis(cyanovaleric acid),2,2′-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide},2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide],2,2′-azobis(N,N′-dimethyleneisobutyramidine),2,2′-azobis(N,N′-dimethyleneisobutyramidine)dihydrochloride,2,2′-azobis(2-amidinopropane)dihydrochloride,2,2′-azobis-{2-methyl-N-[1,1-bis(hydroxymethyl)-2-ethyl]propionamide},2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide],2,2′-azobis(isobutyramide)dihydrate, and derivatives thereof.

Initiators which have an appreciable solubility in a more hydrophobicreaction medium typically include oil soluble initiators such as azocompounds exemplified by the well known material2,2′-azobisisobutyronitrile. Other readily available initiators are acylperoxides such as acetyl and benzoyl peroxide as well as alkyl peroxidessuch as cumyl and t-butyl peroxides. Hydroperoxides such as t-butyl andcumyl hydroperoxides may also be used.

The source of initiating radicals will generally be provided by aninitiator compound per se rather than from the spontaneous generation ofradicals from monomers.

Without wishing to be limited by theory, it is believed that selecting aparticular mole ratio of RAFT agent to initiator may also facilitate inproviding for PAN-based polymers having a high M_(n) and a lowdispersity.

In a further embodiment, the method of the invention further comprisesinitiating the polymerisation using an initiator compound, wherein themole ratio of the RAFT agent to initiator compound is no more than about3, or no more than 2.8, or no more than about 2.5, or no more than about2.2, or no more than about 2. In a further embodiment, the mole ratio ofRAFT agent to initiator compound ranges from about 0.5 to about 3, orfrom about 1 to about 3.

In another embodiment the initiator compound is an azo compound such as2,2′-azobis(isobutyronitrile) (AIBN).

The method according to the invention can advantageously be performedusing conventional RAFT polymerisation techniques and equipment. Thepolymerisation may be performed in solution (e.g. using a suitablesolvent), or as a suspension or emulsion polymerisation.

In one embodiment, the polymerisation is a solution polymerisationconducted in a suitable reaction medium or solvent. Without wishing tobe limited by theory, it is believed that the selection of a suitablereaction medium or solvent may also facilitate providing for PAN-basedpolymers having a high M_(n) and a low dispersity.

In one embodiment, the polymerisation is performed in a reaction mediumselected from dimethylsulfoxide (DMSO), dimethylformamide (DMF),dimethylactetamide (DMA), ethylene carbonate (EC), propylene carbonate(PC) or combinations thereof.

In another embodiment, the polymerisation is performed in a reactionmedium selected from dimethylsulfoxide (DMSO), ethylene carbonate (EC),propylene carbonate (PC) or combinations thereof.

In a further embodiment, the polymerisation is performed using PC as thereaction medium. Surprisingly, PC has been found to not only provide forPAN-based polymers having a very high M_(n) and a low dispersity, butthe % conversion of monomer into polymer has also been excellent. PC isalso advantageously relatively non-toxic.

In addition to factors such as the polymerisable monomer to RAFT agentmole ratio, the selection of the RAFT agent, the RAFT agent to initiatea compound mole ratio and the selection of reaction medium or solvent,without wishing to be limited by theory it is believed that theselection of polymerisation temperature may also facilitate providingfor PAN-based polymers having a high M_(n) and a low dispersity.

Accordingly, in a further embodiment, the polymerisation is conducted ata temperature of no more than 80° C., or no more than 70° C., or no morethan 65° C., or no more than 60° C., or no more than 55° C., or no lessthan about 50° C. In a further embodiment, the polymerisation isconducted at a temperature within the range of 60° C. to 70° C., or 60°C. to 65° C.

The method according to the invention may further comprise forming theresulting PAN-based polymer into fibre. Formation of such fibre can beachieved using techniques known in the art such as wet spinning,electrospinning and melt spinning.

In a further aspect, the present invention provides a method of formingpoly(acrylonitrile) homo- or co-polymer fibre, said method comprisingwet spinning, electrospinning or melt spinning poly(acrylonitrile) homo-or co-polymer according to the invention.

In one embodiment, the PAN-based polymer according to the invention isprovided in the form of fibre.

Fibre comprising PAN-based polymers according to the invention canadvantageously be used as a precursor in the manufacture of carbonfibre.

The present invention therefore also provides a method of producingcarbon fibre, the method comprising carbonising a fibre comprisingPAN-based polymer according to the invention.

PAN-based polymer according to the invention in the form of fibre canconveniently be converted into carbon fibre using techniques known inthe art.

The invention will now be described with reference to the followingexamples which illustrates some preferred embodiments of the invention.However, it is to be understood that the particularity of the followingdescription is not to supersede the generality of the precedingdescription of the invention.

EXAMPLES Reference Example 1

Add 5 mol/L AN and 1.60×10⁻³ mol/L AIBN, together with the specifiedsolvent to a Shlenk flask. The solution was degassed with nitrogen (e.g.about 20 min) to eliminate the dissolved oxygen. The flask was thencharged with nitrogen and sealed. Then, the flask was placed in an oilbath held at a temperature of 60° C. for 12 hours. The reaction mixtureswere diluted with DMSO, and the polymers precipitated in methanol anddried in a vacuum oven at 30-40° C. until constant weight. The resultsin FIG. 1 show how the solvent selection in acrylonitrile polymerisationcan alter the results. It is evident that the solvent used has an effecton the monomer conversion rate and molecular weight of the PAN polymersobtained. EC, DMSO and PC are useful solvents for PAN preparation,resulting in a higher molecular weight and polymer yield than DMF(Dimethylformamide) and DMA (Dimethylacetamide).

Example 2

Add 6 mol/L acrylonitrile (AN) and 2% methyl methacrylates (MMA), andDMSO were added to a Shlenk flask. An initiator AIBN and a RAFT agent4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid were added into thereaction mixture at a molar ratio of 1:2, and AN to the RAFT agent ratiowas 8500:1. The solution was bubbled with nitrogen for about 20 min toeliminate the dissolved oxygen. Then the flask was placed in an oil bathheld at a temperature of 61° C. for 15 hours. The reaction mixtures werediluted with DMSO, and the polymers precipitated in methanol and driedin a vacuum oven at 30-40° C. until constant weight.

The reaction product was analysed by GPC, and the results are:

M_(n)=240072, and PDI=1.13

Similar results were obtained with methyl acrylate as the comonomer.

Example 3

Same procedure as in Example 2 was used in Example 3 except the AN tothe RAFT agent ratio was 10000:1, with 2% MMA+ Itaconic acid ascomonomers, and the polymerisation temperature was 65° C.

The GPC results are: M_(n)=200610, and PDI=1.11

Example 4

Same procedure as in Example 2 was used in Example 4, except that thesolvent used was PC, and AN to RAFT ratio was 9000:1, without acomonomer used.

The GPC results are: M_(n)=465015, PDI=1.15

1. Poly(acrylonitrile) homo- or co-polymer having a number average molecular weight (M_(n)) of at least 200,000 g/mol and a dispersity (

) of less than 1.3.
 2. The poly(acrylonitrile) homo- or co-polymer according to claim 1 having a dispersity (

) no greater than 1.25.
 3. The poly(acrylonitrile) homo- or co-polymer according to claim 1 having a dispersity (

) no greater than 1.20.
 4. The poly(acrylonitrile) homo- or co-polymer according to claim 1 having a number average molecular weight (M_(n)) of at least 450,000 g/mol.
 5. The poly(acrylonitrile) homo- or co-polymer according to claim 1 having covalently bound thereto residue of a dithiobenzoate or trithiocarbonate RAFT agent.
 6. The poly(acrylonitrile) homo- or co-polymer according to claim 5, wherein the dithiobenzoate or trithiocarbonate RAFT agent is cyano functionalised, carboxylic acid functionalised, or cyano and carboxylic acid functionalised.
 7. The poly(acrylonitrile) homo- or co-polymer according to claim 6, wherein the dithiobenzoate or trithiocarbonate RAFT agent is carboxylic acid functionalised.
 8. The poly(acrylonitrile) homo- or co-polymer according to claim 1 in the form of fibre.
 9. The poly(acrylonitrile) homo- or co-polymer according to claim 1 having covalently bound thereto residue of a RAFT agent selected from 1-8 or a RAFT agent precursor compound selected from 9-11:

where the or each R is independently selected from H or CH₃, the or each R′ is independently selected from H, CH₃, or CN, n=0-15, and m=0-10.
 10. The poly(acrylonitrile) homo- or co-polymer according to claim 9, wherein the RAFT agent is 1 or
 5. 11. Carbon fibre comprising carbonised residue of poly(acrylonitrile) homo- or co-polymer according to claim
 1. 12. A method for producing poly(acrylonitrile) homo- or co-polymers by RAFT polymerisation, the method comprising polymerising acrylonitrile and optionally one or more ethylenically unsaturated co-monomers under the control of a RAFT agent, wherein the mole ratio of the polymerisable monomers to the RAFT agent is at least 1,000.
 13. The method according to claim 12, wherein the mole ratio of the polymerisable monomers to the RAFT agent is at least 4,000.
 14. The method according to claim 12 further comprising initiating the polymerisation using an initiator compound, wherein the mole ratio of the RAFT agent to initiator compound is no more than
 3. 15. The method according to claim 12, wherein the polymerisation is conducted at a temperature of no more than 70° C.
 16. The method according to claim 12, wherein acrylonitrile is copolymerised with one or more ethylenically unsaturated co-monomers selected from acrylic acid, methacrylic acid, itaconic acid, allysulfonic acid, maleic acid, crotonic acid, acrylic acid methylester, methacrylic acid ethylester, ammonium itaconate, ammonia acrylate, butyl methacrylate, propyl acrylate, stearyl acrylate, isobutyl methacrylate, methyl methacrylate, ethyl methacrylate, vinyl acetate, methyl acrylate, vinyl chloride, vinylidine chloride, styrene, acylamide, methacrylamide, 3-ammoniumcarboxylate-3-butenoic acid methyl ester, and combinations thereof.
 17. The method according to claim 12, wherein the RAFT agent used is a dithiobenzoate or trithiocarbonate RAFT agent.
 18. The method according to claim 17, wherein the dithiobenzoate or trithiocarbonate RAFT agent is cyano functionalised, carboxylic acid functionalised, or cyano and carboxylic acid functionalised.
 19. The method according to claim 18, wherein the dithiobenzoate or trithiocarbonate RAFT agent is carboxylic acid functionalised.
 20. The method according to claim 12, wherein the RAFT agent used is selected from 1-8 or provided by a RAFT agent precursor compound selected from 9-11:

where the or each R is independently selected from H or CH₃, the or each R′ is independently selected from H, CH₃, or CN, n=0-15, and m=0-10.
 21. The method according to claim 20, wherein the RAFT agent is 1 or
 5. 22. The method according to claim 12, wherein the polymerisation is performed in a reaction medium and the reaction medium is selected from dimethylsulfoxide, ethylene carbonate, propylene carbonate, and combinations thereof.
 23. The method according to claim 12, wherein the resulting poly(acrylonitrile) homo- or co-polymers have a number average molecular weight (M_(n)) of at least 200,000 g/mol and a dispersity (

) of less than 1.25.
 24. A method of producing carbon fibre, the method comprising carbonising a fibre comprising poly(acrylonitrile) homo- or co-polymer according to claim
 1. 25. Poly(acrylonitrile) homo- or co-polymer having a number average molecular weight (M_(n)) of at least 150,000 g/mol and a dispersity (

) of less than 1.25.
 26. Poly(acrylonitrile) homo- or co-polymer having a number average molecular weight (M_(n)) of at least 100,000 g/mol and a dispersity (

) of less than 1.2. 